I used two 6800GTs in SLI mode for a month - one from eVGA and one from XFX. I did have a problem that I could not resolve. I could play for hours with beautiful silky-smooth frame rates, however, when I walked away from the computer, and it flipped the monitor off in 20 minutes for "powersaver" - the only feature I had selected, when I returned, upon executing a game I got instant lockup. I tried and tried to underclock, different video drivers including betas, differing video card BIOS, and I never could resolve this problem.
I'll tell you also that two 6800GTs would heat my entire room up during an hour of 3D gaming by about 10 degrees. No joke.
I finally got fed up in the hot summer weather AND the lockup issue that I replaced them both with a 7800GTX KO. The room doesn't heat up and my case reads 30 degrees cooler F after 2 hours of WoW raiding. The card is only 5-10 degrees cooler, but that AC3 cooler on the KO model does a fantastic job of dissipating the heat.